Wireless power delivery in dynamic environments

ABSTRACT

An adaptive system for efficient and long-range wireless power delivery using magnetically coupled resonators responds to changes in a dynamic environment, and maintains high efficiency over a narrow or fixed frequency range. The system uses adaptive impedance matching to maintain high efficiency. The wireless power transfer system includes a drive inductor coupled to a high-Q transmitter coil, and a load inductor coupled to a high-Q receiver coil. The transmitter coil and receiver coil for a magnetically coupled resonator. A first matching network is (i) operably coupled to the drive inductor and configured to selectively adjust the impedance between the drive inductor and the transmitter coil, or (ii) is operably coupled to the load inductor and configured to selectively adjust the impedance between the load inductor and the receiver coil.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/402,660, filed Nov. 20, 2014, which is a U.S. National Stage ofPCT/US2013/042085, filed May 21, 2013, which is a continuation of U.S.application Ser. No. 13/843,884, filed Mar. 15, 2013 (U.S. Pat. No.8,827,889, issued Sep. 9, 2014), which claims the benefit of ProvisionalPatent Application No. 61/734,236, filed Dec. 6, 2012, ProvisionalPatent Application No. 61/691,127, filed Aug. 20, 2012, and ProvisionalPatent Application No. 61/649,496, filed May 21, 2012, each of which arehereby incorporated by reference in their entireties.

BACKGROUND

Wireless power transfer using inductive coupling is becomingincreasingly popular for consumer electronic devices. Commercialapplications include wireless charging pads, electronic toothbrushes,induction cookers, and electric car battery chargers. However, none ofthese applications enable the range or geometric freedom that the termwireless power suggests. Charging pads and electric toothbrushes requirethat the device be placed very close to (or directly on top of) thecharging pad. This is because the efficiency for traditional inductivelycoupled wireless power transfer systems drops off rapidly as thedistance between the transmitter and receiver increases.

Far-field wireless power transfer techniques use propagatingelectromagnetic waves and are capable of delivering power to a muchlarger volume of space. However, there is an inherent tradeoff betweendirectionality and transfer efficiency. For example, radio frequency(RF) broadcast methods—which transmit power in an omni-directionalpattern—allow for power transfer anywhere in the coverage area. Althoughmobility is maintained, end-to-end efficiency is lost because the powerdensity decreases with the square of the distance. Microwave systemswith high gain antennas have been used to transfer power over severalkilometers at efficiencies of over 90%. However, these systems sufferfrom the need for sophisticated tracking and alignment equipment tomaintain a line of sight (point-to-point) connection.

Regulatory restrictions limit the amount of power that can betransmitted in uncontrolled environments for safety as well as emissionsand interference reasons. As a result, the main commercial use offar-field wireless power transfer is for passive (i.e., battery free)UHF RFID tags which are limited to four watts equivalent isotropicradiated power in the USA.

Recent research efforts using magnetically coupled resonators (MCRs) forwireless power transfer have demonstrated the potential to deliver powerwith more efficiency than far-field broad-cast approaches, and at longerranges than traditional inductively coupled methods. These techniquesuse high quality factor (“high-Q”) coupled resonators that transferenergy via magnetic fields that do not strongly interact with the humanbody. U.S. Patent Publication No. 2012/0153738, to Karalis et al., andU.S. Patent Publication No. 2012/0080957, to Cooper et al., both ofwhich are hereby incorporated by reference in their entireties, disclosecertain aspects of wireless energy transfer using MCRs.

However, a drawback of current MCR systems is the inability toefficiently adapt to changes in the environment. For example,unpredictable loads and changes in distance and orientation between MCRcoils rapidly change system operating points, which disrupt theend-to-end wireless power transfer efficiency. Dynamic adaptation of asystem to these types of events is a critical capability in developingfully functional and versatile wireless power solutions.

FIG. 1 shows a diagram of a basic prior art wireless power system 90using high-Q MCRs. A transmitter module 91 includes a single turn driveloop 93 and a multi-turn, spiral resonator or transmit coil (Tx coil)94. When an RF amplifier 92 drives current through the drive loop 93 atthe transmitter module's 91 resonant frequency, the resultingoscillating magnetic field excites the Tx coil 94. The Tx coil 94 storesenergy in the same manner as a discrete LCR tank. This results in alarge oscillating magnetic field in the vicinity of the Tx coil 94. Ahigh-Q coil implies that more energy can be stored on the coil, whichalso results in greater magnetic flux density at a given point in space.

The receiver module 95 is designed similarly. It includes a multi-turn,spiral resonator or receive coil (Rx coil) 96 and a single turn loadloop 97, which is connected to an end device 98. The drive loop 93 andTx coil 94 are magnetically coupled, and the load loop 97 and Rx coil 96are magnetically coupled. Similarly, the Tx coil 94 and the Rx coil 96share a mutual inductance, which is a function of the geometry of thecoils 94, 96 and the distance between them. The high-Q Tx and Rx coils94, 96 form a single system of coupled resonators, which can efficientlytransfer energy back and forth.

In generally and other parameters being held constant, the couplingcoefficient between the Tx coil 94 and the Rx coil 96 is inverselyproportional to the distance between the coils 94, 96. At relativelyshort distances (in the over-coupled regime) high efficiency powertransfer between the coils 94, 96 can be achieve over a wide frequencyrange. As the separation distance increases, the coupling between theresonators 94, 96 decreases, and the frequency range for high efficiencypower transfer narrows, until the optimal frequency converges to thefundamental frequency of the system (critical coupling). In theover-coupled regime, the resonators 94, 96 share more magnetic flux thanis required to source the load. However, as discussed below, propertuning techniques will enable near constant power transfer efficiencysubstantially within the entire over-coupled regime.

In the under-coupled regime, the shared flux falls below a criticalpoint. Below this point, the Tx coil 94 needs to emit more power tomaintain the magnetic field than can be absorbed by the Rx coil 96. Theresult is that maximum efficiency cannot be achieved. Critical couplingis the point of transition between these two regimes and corresponds tothe greatest range at which maximum efficiency can still be achieved.The under-coupled regime is still capable of wireless power transfer,but efficiency decreases rapidly as distance increases.

A system is disclosed that takes advantage of the over-coupled regime tocreate a volume of space providing high efficiency power transferbetween the transmitter module 91 and the receiver module 95, towirelessly provide power to the end device 98. The system has also beenfound to provide range extension in the under-coupled region.

The coupling coefficient between the Tx coil 94 and the Rx coil 96depends of operating frequency. Prior art systems have proposedmaintaining high efficiency in transferring energy in an MCR systemusing dynamic frequency tuning. The goal of dynamic frequency tuning isto automatically adjust the transmitter frequency (e.g., amplifier 92)to provide maximum power transfer efficiency between the Tx coil 94 andthe Rx coil 96, e.g., as a user moves the Rx coil 96 within the system'sworking range.

The mutual inductance between the Tx coil 94 and the Rx coil 96 is afunction of the coil geometry and the distance and orientation betweenthe coils 94, 96. Although it is possible to transfer wireless powerwithout adaptive techniques, small changes in distance between thetransmitter and the receiver will generally cause very large changes inefficiency. However, by dynamically adapting the amplifier 92 frequency,a relatively large region of space can be accommodated for highefficiency power transfer.

However, in many applications adaptive frequency tuning is not a viableapproach for high efficiency power transfer, in part because ofgovernmental regulation of the frequency spectrum. Narrow bandwidthoperation is desirable for regulatory reasons. Spectrum use regulationsvary from country to country. Currently no country has allocatedspectrum specifically for wireless power transfer. However, Industrial,Scientific, and Medical (ISM) bands are allocated internationally for RFapplications other than communication. ISM bands are currently used forapplications such as RF heating and microwave ovens. Therefore, they area natural choice for today's wireless power transfer systems.

The ISM bands are governed in the U.S. by Part 18 of the FederalCommunication Commission (FCC) rules. Part 15 of the FCC rules coverscommunication, even if the communication occurs in an ISM band. Thefield strength limits of Part 15 are more stringent than those of Part18. Therefore, it may be desirable for wireless power transfer systemsnot to use the same band for power transfer and communication.

Existing ISM bands are too narrow to accommodate frequency tuning. Forexample, in a particular test system the bandwidth requirements ofdynamic frequency tuning exceed the available bandwidth from FCCregulations by three orders of magnitude.

The present invention includes methods and systems for an MCR powertransfer system that dynamically adapts to variations in range,orientation, and load using both wide-band and fixed-frequencytechniques. In particular, impedance matching methods and systems aredisclosed that are suitable for fixed frequency operation, adaptivefrequency tuning for wider bandwidth systems, and adaptive load matchingtechniques utilizing maximum power point tracking.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

An adaptive impedance matching wireless power transfer system includes adrive inductor configured to receive RF power, a first high-Q resonatorcoil inductively coupled to the drive inductor, a second high-Qresonator coil inductively coupled to the first high-Q resonator coil,and a load inductor inductively coupled to the second high-Q resonatorcoil. A first matching network, for example a π-match network or anL-match network, is operably coupled to either the drive inductor or theload inductor, and is configured to selectively adjust the impedancebetween the drive or load inductor and the corresponding resonator coil.In an embodiment the drive or load inductor is the inductor for thefirst matching network. In an embodiment the drive inductor and/or theload inductor comprise a single loop.

In an embodiment the first matching network is a π-match network withvariable capacitances, which may be implemented, for example with one ormore banks of capacitors configured in a switchable network. In anembodiment the switchable network is controlled with a microcontrollerthat selectively engages one or more of the capacitors in the bank ofcapacitors, to thereby adjust the impedance between the inductor and theresonator coil.

In an embodiment, the microcontroller adjusts the capacitors to maximizethe forward transmission gain to the transmitter coil, for example usingan exhaustive search through available switch combinations, using lookuptables correlating a measurable parameter of the system, or using ameasured performance parameter of the system.

In an embodiment the system further comprises a second π-match network,wherein the first matching network is operably connected to the driveinductor and the second matching network is operably connected to theload inductor.

In an embodiment the system further comprises a rectifier with an activeimpedance matching circuit configured to receive direct current from therectifier, and a microcontroller configured to monitor the directcurrent from the rectifier and to control the active impedance matchingcircuit to selectively harvest power from the rectifier and providepower to a device.

An adaptive impedance matching wireless power transfer system includes atransmit side comprising a drive inductor configured to receivealternating current electric power from a power source at a fixedfrequency, and a high-Q transmitter coil inductively coupled to thedrive inductor, and a receive side comprising a high-Q receiver coilconfigured to be inductively coupled to the transmitter coil, and a loadinductor inductively coupled to the receiver coil. A first matchingnetwork comprising a plurality of capacitors interconnected to form aswitchable bank of capacitors, and a microcontroller operably connectedto the switchable bank of capacitors, wherein the microcontroller isconfigured and operable to receive a measured operating parameter of theadaptive impedance matching wireless transfer system and to use themeasured parameter to selectively adjust the impedance between either(i) the drive inductor and the transmitter coil, or (ii) the loadinductor and the receiver coil.

In an embodiment the measured parameter comprises an S-parameter or anRMS voltage measured in the system.

In an embodiment the measured parameter is measured on the transmitside, and the microcontroller selectively adjusts the impedance betweenthe drive inductor and the transmitter coil.

In an embodiment the measured parameter is measured on the receive side,and the microcontroller selectively adjusts the impedance between theload inductor and the receiver coil.

In an embodiment the measured parameter is measured on the receive side,and the microcontroller selectively adjusts the impedance between thedrive inductor and the transmitter coil.

In an embodiment the measured parameter is measured on the transmitside, and the microcontroller selectively adjusts the impedance betweenthe load inductor and the receiver coil.

In an embodiment the first matching network is operably connected to thetransmit side, and further comprising a second matching networkcomprising a plurality of capacitors interconnected to form a switchablebank of capacitors, and a second microcontroller operably connected tothe switchable bank of capacitors, wherein the second microcontroller isconfigured and operable to receive a measured operating parameter of theadaptive impedance matching wireless transfer system and to use themeasured parameter to selectively adjust the impedance between the loadinductor and the receiver coil.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a basic prior art wireless power system usingmagnetically coupled resonators;

FIG. 2A is a block diagram illustrating schematically a wireless powersystem for dynamic impedance matching in accordance with the presentinvention;

FIG. 2B is a block diagram similar to that shown in FIG. 2B, wherein thedrive loop and load loop function is provided by the inductors in thematching networks;

FIG. 3 illustrates a method for optimizing a wireless power system, forexample that shown in FIG. 2A, for narrow band or single frequencyoperation;

FIG. 4 is a circuit diagram for implementing a π-match network suitablefor the system shown in FIG. 2A;

FIG. 5 is a diagram illustrating an adaptive rectifier system inaccordance with the present invention, that is suitable for use with thesystem shown in FIG. 2A or for a power transfer system without adaptiveimpedance matching; and

FIG. 6 is a block diagram illustrating another embodiment of a wirelesspower system in accordance with the present invention.

DETAILED DESCRIPTION

A system and method for the wireless power transmission that takeadvantage of the unique properties of magnetically coupled resonators(MCRs) is disclosed. A detailed description of the operating principlesand performance characteristics of MCRs is presented in “Analysis,Experimental results, and range adaptation of magnetically coupledresonators for wireless power transfer,” A. Sample, D. Meyer, and J.Smith, Industrial Electronics, IEEE Transactions on, Vol. 58, No. 2, pp.544-554, February 2011, which is hereby incorporated by reference in itsentirety. A brief overview of system features that can enable seamlesswireless power delivery is provided to facilitate an understanding ofthe present invention.

A wireless power transfer system is disclosed that uses high-Qmagnetically coupled resonators, and one or more dynamic impedancematching networks to maintain high power transfer efficiency between theresonators within a very narrow frequency band, or at a singlepredetermined frequency.

It will be appreciated that the input impedance of the prior art MCRwireless power system 90 shown in FIG. 1 will vary due to changes in thelocation and/or orientation of the Tx and Rx resonator coils 94, 96because the mutual inductance between the Tx coil 94 and the Rx coil 96varies as a function of distance and orientation. Additionally, when theTx and Rx coils 94, 96 are sufficiently close to each other, the crosscoupling and direct capacitance feed through from one coil can detunethe opposite coil and reduce the quality factor Q of each MCR. Both ofthese factors contribute to a mismatch between source and load impedancethat substantially degrades power transfer efficiency.

With the system 90 the detuning effect or drop in efficiency may beovercome by varying the loop-to-coil geometry, and therefore thecoupling coefficient k_(lc). However, this method of tuning k_(lc) isnot preferred because it requires mechanically adjusting the distancebetween each loop 93, 97 and its corresponding coil 94, 96.

The present inventors disclose a method and system for achieving highefficiency narrowband operation by adding dynamic impedance matchingnetworks to one or both of the drive loop 93 and the load loop 97. Ablock diagram of a wireless power system 100 for dynamic impedancematching is shown in FIG. 2A.

FIG. 2B is a block diagram illustrating an alternative system 100′wherein the drive loop 103 and the load loop 107 are functionallyreplaced by the π-match networks 101, 109, respectively, with thecorresponding inductor Lπ1, Lπ2 serving at the drive and/or loadloop(s).

In the wireless power system 100 a first or Tx adjustable π-matchnetwork 101 is provided between an amplifier 102 and a drive loop 103that is magnetically coupled to a high-Q MCR Tx coil 104. A second or Rxadjustable π-match network 109 is provided between a load loop 107coupled to an MCR Rx coil 106 and an end device 108. In this embodiment,the topology includes variable capacitors C_(S1), CL1 and a fixedinductor Lπ1 with the parasitic equivalent series resistance r_(p)) onthe transmit side, and variable capacitors C_(S2), C_(L2) and a fixedinductor Lπ2 on the receive side. The transmit side inductor Lπ1 (forthe first adjustable π-match network 101) and the receive side inductorLπ2 (for the second adjustable π-match network 109) may have differentinductance values.

This wireless power system 100 performs dynamic impedance matching bydynamically controlling the variable capacitances of both π-matchnetworks 101, 109. Other matching networks, for example L-matchnetworks, may alternatively be used and are contemplated by the presentinvention. However, compared to other matching network topologies theπ-match network is currently preferred for adaptive wireless powertransfer. The π-match network has several advantages, for example theπ-match network uses a fixed-value inductor in the high-current path,and variable capacitors that handle relatively low power in shuntconfigurations. Also, the π-match network is able to match sourceimpedances that are both greater than, equal to, and less than loadimpedances.

Although FIGS. 2A and 2B show matching networks on both the Tx side andthe Rx side, it is contemplated that a system may be implemented with amatching network on only one side. It is a design consideration whetherto place a matching network on both the Tx and Rx sides. The combinationof the Tx π-match network 101 at the input to the drive loop 103 and theRx π-match network 109 at the output from the load loop 107 provides awider range of impedance matching between source and load impedancesthan would be available with either network 101, 109 alone, thusresulting in higher wireless power transfer efficiency at a singlefrequency for any separation distance. This is because in many instancesπ-match networks at both sides can do a better job of impedance matchingwhen there is a large deviation between source and load terminationimpedances. A π-match network has an extra degree of freedom from thetypical L-match network, and that is the Q factor of the matchingnetwork, which can be tuned to achieve a wideband or narrowbandimpedance match. In the L-match network, the Q factor of the matchingnetwork is fixed for a given impedance and capacitance. In a π-matchnetwork the same impedance match can be achieved for wide range ofmatching network Q factors.

Using unconstrained nonlinear optimization to determine the idealcapacitor values for π-match networks 101, 109 that will maximize theforward transmission gain S21 are determined for a range of couplingcoefficients between the MCR coils 104, 106. The current method measuresone or more of the scattering parameters, or S-parameters ([S] matrices)for one or both of Lπ1, Lπ2 and for the set of MCR coils 104, 106, andconverts the S-parameters into ABCD-matrices, as is known in the art fortwo-port network analysis. The ABCD representation is convenient becausea series of cascaded two-port networks can be modeled by computing theproduct of their individual ABCD matrices to form a single lumpedABCD-matrix for the system. The ABCD matrices for the Tx π-match network101, the MCR coils 104, 106 and the Rx π-match network 109 aremultiplied together. After converting the lumped ABCD-matrix back to anS-matrix, the source and load capacitor values in each π-match network101, 109 are determined by selecting values that optimize [S21] at thedesired frequency.

The method will now be described in more detail with reference to FIG.3, for the system shown in FIG. 2A. The [S] matrices and the [Y]matrices for the components are obtained 200. The S-parameters for theset of MCRs may be obtained, in a number of ways, including for example,from manufacturer data, with a vector network analyzer, with adirectional coupler, or the like. It is preferable to use measured dataso that all of the parasitic effects are considered. Typically, thetransfer functions for a 4-coil MCR system neglect parasitic effectssuch as cross-coupling and coil de-tuning that can significantly reduceefficiency at the resonant frequency. The admittance matrices [Y] arealso defined for the capacitance components of the π-match networks 101,109. Obtaining the [S] matrices and [Y] matrices is well within theabilities of persons of skill in the art.

The [S] and [Y] matrices are converted into [ABCD] transmission matrices202. These [ABCD] matrices for the individual component are combined204, e.g., by multiplying the cascaded [ABCD] matrices to define asystem [ABCD] matrix. A system [S] matrix is calculated 206 from thesystem [ABCD] matrix using complex termination impedances to match asource impedance to a defined load impedance. Finally, a conventionalconstrained non-linear optimization algorithm may be used to determinethe component values C_(S1), C_(L1), C_(S2), C_(L2) in each network 208that maximize S21. Equivalently, the algorithm may minimize thereflection S-parameter, S11. It is also contemplated that the algorithmmay be configured to maximize power transfer efficiency, if data from anout of band radio is available to communicate between the power transmitside and receive side.

FIG. 4 illustrates an exemplary switching circuit that is suitable forimplementing the first and/or second π-match networks 101, 109 shown inFIG. 2A. In this embodiment the variable capacitor C_(S1) (or C_(S2)) isimplemented with a plurality of fixed capacitors C1-C7, and the variablecapacitor C_(L1) (or C_(L2)) is implemented with a plurality of fixedcapacitors C9-C15, wherein the capacitors C1-C15 are networked inswitchable capacitor banks 150, 152. Each of the capacitors C1-C15 inthe capacitor banks 150, 152 are selectively engaged through a networkof controllable micro-switches M1-M16. A microcontroller 154 isconfigured to engage the desired capacitors, which are selected toapproximately maximize S21.

In the example switching circuit shown in FIG. 4 the microcontroller 154controls the capacitor banks 150, 152 through the switching circuit,which includes the microcontroller 154. The microcontroller 154 isoperably connected to control a plurality of sub-circuits that engage anassociated one of the capacitors C1-C15. The switching circuit isconfigured to selectively engage or disengage the associated one of theplurality of capacitors C1-C15 from the capacitor banks 150, 152. Theswitching circuit will be described with reference to engaging anddisengaging capacitor C1. In this embodiment the controllablemicroswitches M1-M16 are field effect transistors, for example MOSFETs(metal-oxide semiconductor field-effect transistors). Each sub-circuitalso includes a pair of back-to-back field effect transistors (e.g., M1and M2) that connect the associated capacitor (e.g., C1) to a groundGND. The sub-circuits include a gate drive filter (e.g., R1, D1, Q1, R2)that is operatively controlled by the micro-processor 154 and configuredto selectively switch the field effect transistors (e.g., M1, M2)between an open state and a closed state. In particular, themicrocontroller 154 is configured to receive a measured operatingparameter of the adaptive impedance matching wireless power transfersystem (for example, an S-parameter, as described above) and to use themeasured operating parameter to selectively adjust the impedance betweenthe inductor Lπ and the associated MCR coil 104, 106 (FIG. 3).

In practice, it may be time consuming to determine the optimal valuesusing the algorithm illustrated in FIG. 3. In an alternative embodiment,using the capacitor banks 150, 152 described above, a control algorithmmay exhaustively sweep each possible combination of capacitor settingswhile monitoring one or more of the scatter parameters, and select theconfiguration that achieves minimum reflected power. For example, tenswitchable shunt capacitors (five on each side of the inductor) have atopology with 1,024 possible states.

It is also contemplated, and will be apparent to persons of skill in theart, that other approximate methods may be selected to arrive at anoptimal set of capacitor settings, in order to achieve more rapidswitching in a dynamic environment. For example, the control algorithmmay be configured to intelligently estimate the coupling coefficientbetween the two MCR coils, for example, by detecting the distancebetween the coils 104, 106. A table of the optimal component valuesrepresenting the possible physical arrangements between the two MCRcoils 104, 106 may be pre-calculated, and the physical positioning ofthe MCR coils may be used with a lookup table to control the optimalcapacitor bank 150, 152 settings.

In another embodiment the power delivered to the load 108 (FIG. 2A) maybe monitored at the receive side of the system, and an out of band radiolink (not shown) may be used to report back to the control algorithm atthe transmit side the status of the received power. The system may thenautomatically detect a change in distance or orientation between theMCRs 104, 106. Such changes could then be used to initiate a new sweepthrough the switch settings, i.e., when a change in the couplingcoefficient is detected. In yet another embodiment, rather than anexhaustive sweep through the switching network the control algorithm mayuse a gradient approach to select only a subset of possible capacitorsettings to find a local optimal transfer efficiency.

A significant challenge in developing effective wireless power systemsis the efficient rectification of RF to DC power across the systemsoperating points. This issue arises from the desire to maintain optimalimpedance matching between the receiving antenna and the rectifier asthe impedance of the load for the application is changing. To maintainoptimal power transfer while undergoing changes in the couplingcoefficient between the MCR coils 104, 106 (which is affected by thedistance and orientation between the source and the load, and byfluctuations in the load), an adaptive rectifier has been developed thatuses a nonlinear impedance matching circuit element and control methodto adapt to changes in the environment.

A diagram of the adaptive rectifier system 120 is shown in FIG. 5. Inthis exemplary embodiment, the power 122 from the Rx side second π-matchnetwork 109 (FIG. 2A) is provided to a full wave rectifier 124 thatconverts the RF power 122 to DC power. A dynamic impedance matchingcircuit 125 is controlled by a microcontroller 126 that receives inputfrom conventional voltage and current sensing circuits 128 and generatesa pulse width modulated (PWM) control signal 127. The impedance matchingcircuit 125 uses a feed-forward buck converter 130 to control the ratioof voltage to current that is drawn from the rectifier 124 and deliveredto the load 132. Additional control algorithms and/or voltage regulationstages may be provided for a particular application.

The adaptive rectifier system 120 architecture and the controlalgorithms implemented on the microcontroller 126 is similar to MaximumPower Point Tracking (MPPT) techniques used for harvesting maximum powerfrom solar cells. See, for example, U.S. Pat. No. 7,986,122, to Fornageet al., which is hereby incorporated by reference.

In a wireless power transfer system such as the system 100 describedabove (FIG. 2A), the output of the MCR Rx coil 106 and the π-matchnetwork 109 presents a variable source resistance. The typicalapplication or load 108 will also present a variable load resistance.Thus adaptation techniques are beneficial to optimize power transfer.

The adaptive rectifier system 120 may comprise a full wave rectifier124, over voltage protection (not shown), a high voltage synchronousNMOS driver (e.g., a high voltage synchronous N-channel MOSFET driver,such as the LTC® LTC4444 MOSFET driver available from Linear TechnologyCorporation, in Milpitas, Calif.), circuits for measuring voltage andcurrent 128, and an microcontroller 126 that implements the controlalgorithm for tracking the maximum power point of the rectifier 124(e.g., the MSP430™ ultra-low-power microcontroller available from TexasInstruments Incorporated, in Dallas, Tex.).

One commonly overlooked aspect of RF rectifier design is that the loadimpedance of the application is essentially transferred through therectifier and impacts the impedance match between the RF antenna/coilsand the input of the rectifier itself. Occasionally this apparent powerloss to the load is interpreted as inefficiencies in the rectifier.However, the present inventors believe RF power is being reflected offof the rectifier-antenna interface.

For example, consider an RF amplifier connected to an ideal rectifierthat is terminated into a 200 Ω load resistor. The ideal rectifier willnot alter the ratio of voltage to current (characteristic impedance)passing through it, but will simply invert the negative portion of theincoming sine wave. Thus, when looking into the rectifier, the impedanceseen is simply that of the 200 Ω resistor. Therefore, if the rectifieris driven by a source with 50Ω characteristic impedance, a portion ofthe incident wave will be reflected due to the mismatch between the 50 Ωto 200 Ω interface, resulting in an apparent power loss to the load.From this example it is clear that the loading conditions placed on therectifier make a significant impact on the total power delivered to theload.

To illustrate the issue of load matching and to demonstrate theeffectiveness of the new adaptive rectifier and the improvement madewhen the adaptive rectifier is enabled, an experiment was performedwherein the RF amplifier 102 (FIG. 2A) with a source impedance of 50 Ωis connected to the adaptive rectifier 124. The RF amplifier 102 sweepsits output power from 3-30 watts at a fixed frequency of 13.56 MHz. Ateach sweep point, an electronic load provided a second sweep of loadcurrent, which emulated different power consumption modes that anapplication might present. The resulting rectifier 124 output voltagesand currents were recorded using a digital multimeter. A host computerrunning Labview® was used to control the system and record data.

When rectifier adaptive impedance matching 125 is turned off it wasobserved that under some loading conditions applied to the rectifier 124an impedance mismatch occurs between the output of the coils and theinput of the rectifier 124, and this mismatch results in poor powertransfer. There is only a narrow operating range where optimal powertransfer can be achieved.

When the adaptive impedance matching circuit 125 is enabled, the MSP430microcontroller 126 measures the output voltage and current ratiodelivered to the load 132. The control algorithm adjusts the PWM signal127 that drives the feed-forward buck converter 130. This maximizesrectified power and thus maximizes the amount of power delivered to theload 132. For nearly any input power level and load current, anoperating point can be found that maximizes power transfer, whichresulted in a plateau of near constant transfer efficiency. Theconclusion is that rectifiers that use MPPT techniques can effectivelymitigate load variation, which would normally disrupt power transfer.

The above describes a method for controlling the apparent load impedanceseen by the output of the rectifier 124 to optimize the RF powertransfer. In effect, the loading condition on the rectifier 124maintains the optimal impedance match between the input of the rectifier124 and the output of the RF amplifier 102.

Another way to look at the system 100 is that if the source impedance ofthe amplifier 102 (or magnetically coupled resonators) is not 50Ω, themaximum power point tracking algorithm on the microcontroller 126 willstill servo the PWM control signal 127 to maximize the power transfer.This will in turn change the input impedance to the rectifier 124 toclosely match the output impedance of the amplifier 102. Thus theadaptive matching circuit block 125 can be used to control the realinput impedance of the rectifier 124.

Controlling the duty cycle of the feed-forward buck converter 130 allowsthe adaptive rectifier to servo its input impedance. However, somereactance is introduced and the impedance matching is not purely real.This is believed to be due to the junction capacitance of the diodes.One possible improvement to the system, therefore, would be to mitigatethis parasitic reactance with a switched impedance matching network.Ultimately, this shows that using a feed-forward buck converter 130 toform an adaptive rectifier is an effective means of electronicallycontrolling the RF impedance of a rectifier 124 using only solid statedevices.

Another embodiment of a system 140 for adaptive wireless power transferusing MCRs shown in FIG. 6, which includes the MCR system shown in FIG.2A, with π-match networks 101, 109 on both the Tx side and the Rx side.The system 140 includes a transmitter board 142 with a digital signalprocessor (DSP) 144 (e.g., a TMS320® DSP available from TexasInstruments Incorporated). The DSP 144 controls all peripherals on thetransmitter board 142 and communicates with an external PC via aserial-to-USB chip (not shown). To detect how much power the systemdelivers to the load 132, the incident and reflected ports of adirectional coupler 146 are attached to the inputs of an RF detectorchip 148. The detector chip 148 outputs a voltage that is proportionalto the log magnitude ratio and phase between incident and reflectedpower (i.e., 1/S11). For example, if the DSP 144 is clocked at 150 MHzit may take many digital samples in a short period of time. In fact, itonly takes this system 140 about 5 μs to take one data point.

Using these measurements, the DSP 144 adjusts the transmit frequency ofan RF synthesizer 151, which drives the amplifier 102 through a low-passfilter 153. Optionally, the system 140 may also employ dynamic impedancematching by controlling π-match boards 101, 109, for example, viaparallel general purpose input/output (“GPIO”) interfaces from the DSP144. An external RF amplifier 102 is used to achieve an output power ofup to 100 W in this exemplary embodiment. The receive side includes areceiver board 160 that may incorporate the rectifier system 120 shownin FIG. 5. Optionally, both the transmitter board 142 and the receiverboard 160 include out-of-band radios 149, 169 (e.g., CC2500 transceiversavailable from Texas Instruments Incorporated), which implementout-of-band communication and allow the load to provide informationabout power consumption, position, or orientation, as well as controlfor a remote π-match board.

The system's 140 control algorithm chooses the optimal system parameters(π-match settings) given the current system state and maximizes powertransfer over time as described above.

The system 140 is capable of fixed frequency operation using dynamicimpedance matching. π-match boards 101, 109 contain capacitor banks thatcan be switched on or off by a parallel GPIO interface. The search spacefor actively controlling the π-match networks 101, 109 is morecomplicated than that of frequency tuning. Where frequency tuning'ssearch space was one-dimensional, the space for impedance matching istwo-dimensional, as the system can change both the Tx-side or Rx-sidecapacitances. Thus, the bank capacitor values should be chosen toprovide the most effective matching circuit with the fewest number ofcapacitors. It is contemplated that for any given arrangement of MRCcoils 104, 106 it may be determined that some capacitor settings willnot correspond to optimal impedance matches, and may be excluded fromthe search space ahead of time.

Wireless power systems based on magnetically coupled resonators canrealize the vision of seamless, reliable wireless power delivery if theyare able to adapt to variations in range, orientation, and loadingconditions. The key insight is that the over-coupled regime allows forhigh efficiency and near constant power delivery if the system is tunedproperly.

In particular, we have demonstrated that adaptive impedance matchingtechniques used for fixed frequency operation can enable wireless powerdelivery to larger areas of space than previously published work.Additionally we have introduced an adaptive rectifier topology that iscapable of adapting to changes in loading conditions to allow foroptimal power delivery to the load. Conversely, the adaptiverectification technique also allows a receiver to control its inputimpedance to ensure proper matching to the magnetically coupledresonators. Finally, a full end-to-end system capable of adapting toreal-time changes in the environment while maintaining optimumefficiency is disclosed.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An adaptive impedancematching wireless power transfer system comprising: a drive inductorconfigured to receive alternating current electric power from a powersource at a fixed frequency; a high quality factor (“high-Q”)transmitter coil inductively coupled to the drive inductor; a high-Qreceiver coil configured to be inductively coupled to the transmittercoil; and a load inductor inductively coupled to the receiver coil; afirst impedance matching network comprising a plurality of capacitorsthat are (i) operably coupled to the drive inductor and configured toselectively adjust the impedance between the drive inductor and thetransmitter coil, or (ii) are operably coupled to the load inductor andconfigured to selectively adjust the impedance between the load inductorand the receiver coil; a switching circuit comprising a microcontrolleroperably connected to a plurality of sub-circuits, wherein each of theplurality of sub-circuits engage an associated one of the plurality ofcapacitors to operably engage or disengage the associated one of theplurality of capacitors; wherein each sub-circuit comprises: a pair ofback to back field effect transistors that connect the associatedcapacitor to a ground, and a gate driver operatively controlled by themicroprocessor and configured to selectively switch the field effecttransistors between an open state, and a closed state, each gate drivercomprising: a resistor connected to gates of the pair of back to backfield effect transistors, a transistor of the gate driver, and a diodeconnected to the resistor and to the gates of the pair of back to backfield effect transistors on one side and connected to the transistor ofthe gate driver on the other side, wherein the diode is connected toground when transistor of the gate driver is turned ON; and wherein themicrocontroller is configured to receive a measured operating parameterof the adaptive impedance matching wireless power transfer system and touse the measured operating parameter to selectively adjust the impedancebetween the drive inductor and the transmitter coil or selectivelyadjust the impedance between the load inductor and the receiver coil. 2.The system of claim 1, wherein the drive inductor forms a portion of thefirst impedance matching network.
 3. The system of claim 1, wherein theload inductor comprises a single loop.
 4. The system of claim 1, whereinthe first impedance matching network comprises a first π-match network.5. The system of claim 1, wherein the microcontroller is controlled toselectively adjust the impedance to achieve a capacitance that maximizesthe forward transmission gain to the transmitter coil.
 6. The system ofclaim 1, wherein the microcontroller exhaustively engages eachcombination of the plurality of capacitors to produce an optimal powertransfer.
 7. The system of claim 1, wherein the microcontroller uses themeasured operating parameter with a lookup table to selectively adjustthe impedance.
 8. The system of claim 1, wherein the microcontrollermonitors a measured performance parameter of the system and uses themeasured performance parameter to selectively adjust the impedance. 9.The system of claim 1, wherein the microcontroller monitors more thanone measured operating parameters of the system, and calculates anoptimal capacitance based on the measured operating parameters.
 10. Thesystem of claim 1, wherein the at least one switchable bank ofcapacitors comprises at least five capacitors.
 11. The system of claim1, further comprising a second impedance matching network that isoperable to selectively adjust the impedance between the receiver coiland the load inductor.
 12. The system of claim 11, wherein the loadinductor forms a portion of the second impedance matching network. 13.The system of claim 11, wherein the first impedance matching networkcomprises a first π-match network with variable capacitances and thesecond impedance matching network comprises a second n-match networkwith variable capacitances.
 14. The system of claim 1, furthercomprising a rectifier configured to receive alternating current fromthe load coil, an active impedance matching circuit configured toreceive direct current from the rectifier, and a microcontrollerconfigured to monitor the direct current from the rectifier and tocontrol the active impedance matching circuit to selectively harvestpower from the rectifier and provide power to a device.
 15. The systemof claim 14, wherein the active impedance matching circuit comprises abuck converter.
 16. The system of claim 15, wherein the microcontrollerselectively adjusts the buck converter.
 17. The system of claim 1,wherein the first impedance matching network comprises a first π-matchnetwork.
 18. The system of claim 1, wherein the measured operatingparameter comprises an S-parameter or an RMS voltage measured in thesystem.